The present invention relates to a brake booster for automotive vehicles comprising a low-pressure casing sealingly subdivided into a low-pressure chamber and a working chamber by an axially movable wall, a reinforcement tube extending axially through the low-pressure casing having its ends fastened to the two end walls of the low-pressure casing and a rolling diaphragm in sealing abutment relative to the movable wall, and a mechanically actuatable control valve to connect the working chamber to the low-pressure chamber or to atmosphere. The housing of the control valve, axially movable in the reinforcement tube, is connected via a push rod with an actuating piston of a master cylinder fastened to the low-pressure casing adjacent the partial vacuum and is connected with radial ribs of the movable wall for movement therewith, the ribs projecting through longitudinal slots of the reinforcement tube, over which slots the rolling diaphragm partially rolls.
In brake boosters for automotive vehicles, it is desired to achieve a light-weight construction and a minimum possible overall length. To provide a construction of the low-pressure casing of minimum possible weight, the force, which occurs upon actuation of the brake and is transmitted from the master cylinder to the point where the brake booster is fastened to the automotive vehicle, for example, to the splashboard, is no longer transmitted via the low-pressure casing, but rather via a central reinforcement tube according to more recent suggestions. Since the control valve housing is disposed within the reinforcement tube and the axially movable wall is required to be urged in a force-transmitting engagement with the control housing, connecting members have to be guided from the movable wall, which is outside the reinforcement tube, through longitudinal slots in the reinforcement tube to connect with the control valve housing. At the same time, there is need for a movable seal between the axially movable wall and the reinforcement tube.
In a brake booster disclosed in a first copending U.S. application of J. Belart and F. Wienecke, Ser. No. 061,113 filed July 26, 1979, assigned to the same assignee as the present application, the longitudinal slots are included in the area of the working chamber, while the sliding seal of the axially movable wall on the reinforcement tube is effected in the area adjoining the longitudinal slots adjacent the low-pressure chamber. Since both the length of the longitudinal slots and the length of the adjoining seal are at least equal to the power stroke of the axially movable wall, the length of the reinforcement tube is required to be greater than double the power stroke of the movable wall, which results in a comparatively large overall length of the brake booster.
A reduced overall length of the brake booster may be achieved according to a second copending U.S. application of R. Becht and P. Bohm, Ser. No. 142,298, filed Apr. 21, 1980, assigned to the same assignee as the present application, by constructing the seal disposed between the movable wall and the reinforcement tube as a rolling diaphragm abutting the reinforcement tube when rolling out, with the rolling diaphram extending over the longitudinal slots. In order to keep the thereby occasioned additional demand upon the rolling diaphram and the resulting wear as small as possible, it is desired to construct the longitudinal slots as narrow as possible. Since the connecting members projecting through the longitudinal slots have to also be constructed accordingly narrow, the connecting members are subjected to a considerable amount of bending stress when the boosting forces are transmitted.
To avoid these disadvantages, it was suggested in a third copending U.S. application of R. Weiler, R. Becht and P. Bohm, Ser. No. 221,584, filed Dec. 31, 1980, assigned to the same assignee as the present application, to arrange the rolling diaphragm in such a manner that it does not roll out over the longitudinal slots, which then will enable the slots and the connecting members to be relatively wide. In this arrangement, however, a space is necessary in the initial or rest position between the front rim of the rolling diaphragm and the beginning of the longitudinal slots, which corresponds to at least half the power stroke of the movable wall.